In the past, an attempt to determine distance between a surface ship and a shallowly submerged submarine depended on direct communication between the ship and the submarine. However due to changing density of water between the ship and the submarine, due to varying thermal, pressure and salinity characteristics of the water, the acquisition range of the submarine by the ship was variable and thus unreliable. Indeed, acoustic waves from the ship might not reach the submarine. Such acoustic waves might be refracted downward into a deep sea, or upward to a surface of a sea, by water between the ship and the submarine. Thus there was no direct reliable acoustic path between the ship and the submarine.
A reliable acoustic path is an acoustic transmission path which is not destroyed by changing thermal, pressure and salinity characteristics of the water between the ship and the submarine. There are reliable acoustic paths vertically, or nearly vertically, in a sea.
In the present invention an array of transponders is used to communicate between a surface ship and a shallowly submerged submarine. The array of transponders is placed near the bottom of the sea, between the ship and the submarine, such that each transponder is on a reliable acoustic path of the ship and such that the submarine is on a reliable acoustic path of each transponder.
In the present invention each of several transmissions is made nearly vertically, between two points in the sea, and thusly on a reliable acoustic path. A transmission is made along a reliable acoustic path from a ship to each of several transponders that are downward of the ship. Another transmission is made along a reliable acoustic path from each of the transponders to the ship. Another transmission is made along a reliable acoustic path from one of the transponders to a submarine that is upward of the transponders. Another transmission is made along a reliable acoustic path from the submarine to each of the transponders that are downward of the submarine. Such reliable acoustic paths are reliable because they are sensitive neither to near-surface-effects nor to the varying losses on reflection that characterize bottom-bounce propagation.
From a point at or near the surface of the sea there are reliable acoustic paths downward. These reliable acoustic paths have comparable transmission losses. For the ship, the reliable acoustic paths that extend downward define an approximately sinusoidal curve, whose upper peaks touch at the surface of the sea beginning at the ship, and whose lower peaks touch the bottom of the sea. For the submarine, the reliable acoustic paths that extend downward define an approximately sinusoidal curve, whose upper peaks touch at the surface of the sea, except for the peak that begins at the submarine, and whose lower peaks touch the bottom of the sea. There is a first such sinusoidal curve for the ship and a second such sinusoidal curve for the submarine.
From a point at or near the bottom of the sea there are reliable acoustic paths upward. These reliable acoustic paths have comparable transmission losses. For each of the transponders the reliable acoustic paths that extend upward, define an approximately sinusoidal curve, whose lower peaks touch the bottom of the sea, except for the peak that begins at a transponder, and whose upper peaks touch the surface of the sea. There is a sinusoidal curve for each of the transponders. These latter sinusoidal curves merge when the transponders are close to each other. These merged curves may be thought of as a third sinusoidal curve.
The first and third sinusoidal curves can be made to overlap, thus producing areas in which the ship and transponders may be positioned whereby the ship can communicate with the transponders and the transponders can communicate with the ship. The second and third sinusoidal curves can be made to overlap, thus producing areas in which the submarine and transponders may be positioned whereby the submarine can communicate with the transponders and the transponders can communicate with the submarine.
The array of transponders used in the present invention allows the position of the submarine, with the respect to the transponders, to be determined by the ship. The transponders of the present invention overcome interference problems, and allow the ship to determine the distance between the ship and the submarine, by using appropriate reliable acoustic paths. The distance between the ship and the submarine can be accurately determined.
The long baseline tracking system of the present invention operates in two steps, also known as two modes. In a first normal mode the distances between a ship and the array of transponders are determined. The position of the ship relative to the array of transponders is plotted. In a second relay mode the distances between the submarine and the array of transponders are determined. The position of the submarine with respect to the array of transponders is plotted. The position of the submarine with respect to the ship may be determined from the plots.